Mongodb – Why does mongodb require three config servers

mongodb

The official mongodb documentation states to use three config servers. They say its for redudancy, because 1 server would be a point of failure…. but why not 2? in a production sharded cluster, if I have 1 shard, I shouldn't need more than 2, right? Is one config server only meant to handle 1 shard? so, if the minimum is 2 shards, then at least I should have 3?

Best Answer

Config servers are a special case, you can use one (for testing), or three and that's it. They are not a replica set, at least not yet, that might change in the future, see SERVER-1448.

Go and (re)read the first two paragraphs here - note the piece about them not being a replica set is explicitly mentioned. I will pick out some other relevant points:

Config servers use a two-phase commit to ensure immediate consistency and reliability

to ensure redundancy and safety in production, you should always use three

The first part gives you a hint about why 3 are needed, the second part is telling, particularly in terms of the safety aspect (redundancy is obvious - you have 3 copies of the data).

With how the 2 phase commit is implemented, all three config servers must receive (and acknowledge) a change for it to be considered successful. That's the safety piece - all three must be in a quorum and have the same data (there are regular checks), and that is why it is three config servers required exactly (for now). When you run with just one for testing, this can't happen, and your cluster meta data is essentially at risk, hence using just one is for testing purposes only.

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